1. Field of the Invention
The invention relates to a device for detecting signaling a physical value of a rim.
2. Description of Related Art
A device of that kind has been known from WO 03/011617 A1. The device described in that publication especially serves for determining and signaling an acceleration encountered at the device, the air pressure and/or the temperature prevailing in a pneumatic tire mounted on the rim. Such a device, which will be described hereafter also as wheel sensor, usually determines the air pressure, acceleration values and/or the temperature at predetermined time intervals, and transmits them at predetermined time intervals equal or greater than the time intervals between the measurements, or else immediately when given threshold values are exceeded, to a receiver and evaluation unit in a vehicle equipped with wheels that are provided with such a wheel sensor. The wheel sensor being covered by the pneumatic tire, it is not visible from the outside. It is not possible to check visually if the sensor is still fastened correctly or if it has got detached from the rim in the course of time. Although it is practically excluded for the wheel sensor to come off the tire, having been screwed to the tire valve in the known manner, screwing a wheel sensor to the tire valve presents a disadvantage insofar as a tire valve of special design is needed, which is more expensive than a conventional tire valve, and in addition a special screw with a through bore is needed, which is more expensive as well. It has, therefore, been considered to fasten wheel sensors on the rim in some different way, for example by means of an adhesive. If a suitable adhesive is selected, and if certain marginal conditions are met, for example if sufficiently large bonding surfaces are selected and carefully prepared for the bonding process, a bonding connection may be sufficiently reliable and durable under driving conditions. Still, it makes sense to consider means and ways of knowing when a wheel sensor should get detached from the rim for one reason or other. If that should happen, the sensor would be flung against the tire inside, by the centrifugal force produced by the driving motion, and would stay there as long as the vehicle continues to move rapidly, or if it reduces its speed, would roll about in an uncontrolled fashion inside the tire, which could damage or destroy the wheel sensor and/or the tire if this condition should persist for some time.
WO 03/011616 A1 therefore suggests to investigate at regular intervals the physical values anyway measured by the wheel sensor, namely the acceleration values encountered at the wheel sensor and/or the temperature, in order to determine if the values so measured contain some information suggesting that the wheel sensor may have got detached from the rim. WO 03/011617 A1 suggests to observe the temperature because that value is clearly position-dependent. WO 03/011617 proposes to observe the acceleration occurring at the wheel sensor because that value may vary atypically when the wheel sensor gets detached from the rim. This proposal is, however, connected with the disadvantage that temperature and acceleration variations occur not only when the wheel sensor gets detached from the rim, but also as a function of the driving style, the road conditions and the moving state of the vehicle, even with the wheel sensor firmly held in place. If the wheel sensor comes off the rim shortly after the vehicle has started to move, it will not be possible to detect that condition by temperature measurements. If accelerating signals are observed, it may be difficult to differentiate between atypical acceleration and acceleration changes occurring in the normal course of driving. Further, when the vehicle is moving at rapid speed, it may be difficult to recognize as atypical a stable acceleration signal provided by a wheel sensor which has got detached from the rim and which at that time is retained on the tire inside by centrifugal force. The proposals known from WO 03/011617, therefore, may lead on the one hand to a false alarm and, on the other hand, to a wheel sensor which may have got detached from the rim being detected not early enough to prevent damage.